This section is intended to introduce various aspects of the art, which may be associated with some of the disclosed embodiments. This discussion is believed to assist in providing a framework to facilitate a better understanding of particular aspects of the disclosed embodiments. Accordingly, it should be understood that this section should be read in this light, and not necessarily as admissions of prior art.
Subwater heat transfer offers substantial benefits for hydrocarbon production including, but not limited to (1) reduced flow assurance concerns, (2) reduced pipeline length and/or line sizing, (3) smaller topside facilities and (4) reduced energy loss from multiphase flow in lines. Subwater heat transfer refers to heat transfer within water where the water comprises, but is not limited to, seawater and/or lake water.
A variety of conventional subwater heat transfer structures exist. One structure includes a box-shaped, completely open-sided structure containing tubes or pipes (i.e., a coil or bundle). The tubes or pipes are parallel with the sea floor and supported at the ends and at numerous locations along their length. Fluid flowing through the tubes or pipes, i.e. process fluid, may be cooled or heated by seawater that enters the structure and flows through voids between neighboring tubes or pipes.
Another conventional subwater heat transfer structure is discussed in U.S. Published Application No. 2010/0252227 (“the '227 application”). The '227 application discloses a subsea cooling unit having an inlet for a hot fluid and an outlet for cooled fluid. The subsea cooling unit comprises coils exposed to seawater and a first propeller for generating a flow of seawater past the coils and through voids between neighboring coils.
Disadvantages of conventional subwater heat transfer structures relate to the velocity of the cooling/heating fluid that flows through the voids in each structure. The velocity of the cooling/heating fluid strongly dictates the thermal performance and size of the structure. The thermal performance of the structure is a function of the velocity of the cooling/heating fluid that flows through the voids. The velocity of cooling/heating fluid in conventional subwater heat transfer structures is not constant and is often small. For example, the cooling/heating fluid velocity may only range from 0.01 to 0.20 m/s. The non-constant nature of the cooling/heating fluid velocity prevents effective, steady-state performance of the structure and effective control of the outlet temperature of the process fluid that is cooled/heated by the cooling/heating fluid. Moreover, the lower velocity of the cooling/heating fluid affects the size of the structure. The lower the cooling/heating fluid velocity, the larger the heat transfer area must be for the structure to achieve a desired thermal performance. Increased cooling/heating fluid velocity (e.g., from 0.01 to 1.00 m/s instead of from 0.01 to 0.20 m/s) can decrease the size of the required heat transfer area by as much as 50 to 60%.
Disadvantages of conventional subwater heat transfer structures also occur when a first propeller is indirectly driven by a second propeller in the outlet for cooled/heated fluid. The indirect connection increases the cost and decreases the reliability of the structure. The indirect connection increases the amount of parts and energy needed to operate the structure and makes the structure more susceptible to system failure.
A need exists for improved technology, including technology that may address one or more of the above described disadvantages of conventional subwater heat transfer structures. For example, a need exists for a subwater heat exchanger that at least one of enhances (i.e., increases) the velocity of the cooling/heating fluid, moves the cooling/heating fluid at a substantially constant velocity, and directly drives the mechanism used to assist cooling/heating the process fluid.